Importing video to sequencer
It ultimately depends on what format and / or where your images originate.

If they originate in a render, the best advice is to store your intermediates as EXR. Even half EXRs are tremendously more useful than many other options.

If your images originate in a camera, you'd be wise to take them to a reasonable high fidelity format such as EXR. This can be tricky depending on your needs and experience and the format of your camera.

If your images originate from a consumer grade camera and the encoded format is H264 or another codec, you can extract the raw frames using various tools such as FFMPEG or its ilk, to varying degrees of quality depending on the tool and options chosen. With proper care, even an H264 encoded file can be properly decoded to either display linear or scene linear EXRs depending on need.

Getting to a high fidelity still image format however, is only a portion of the puzzle. Coming up with a reasonable offline to online path is another challenge entirely, and critical to optimize the quality of your work if you choose to avoid the "good enough" path.